1. Field of the Invention
The present invention is directed to a local antenna for a magnetic resonance apparatus.
2. Description of the Prior Art
Local antennas are utilized in diagnostic magnetic resonance systems because they deliver a better signal-to-noise ratio compared to whose-body antennas. Due to the more uniform intensity profile, the whole-body antenna is often utilized for the excitation of the nuclear spin procession, with only the resulting magnetic resonance signal being received with the local antenna. In addition to local antennas that are fashioned as receive-only antennas, local antennas for transmitting and for receiving are also employed for specific examinations.
Given proper operation, receive-only local antennas are detuned during the transmission phase of the whole-body antenna, so that a coupling of the local antenna with the whole-body antenna becomes minimal, as disclosed, for example, by U.S. Pat. No. 5,144,244. U.S. Pat. No. 5,256,972 discloses a decoupling of the whole-body transmission antenna, by means of radio-frequency switching diodes inserted into the antenna leads. German OS 42 42 592 discloses the use for decoupling of a second winding with an opposite winding direction connected in addition to the usual winding of the reception coil. Transmit/receive local antennas are usually not operated simultaneously with the whole-body antenna; accordingly, they have no possibility for detuning.
German OS 44 40 619 discloses a radio-frequency antenna that wirelessly forwards the received magnetic resonance signals to the magnetic resonance apparatus.
Problems can arise when a local antenna that is improperly connected to the magnetic resonance apparatus is exposed to the transmission field of the whole-body antenna. Even a receive-only local antenna cannot be detuned in this case, and thus the high-frequency currents induced therein can lead to the destruction of antenna components or to an endangerment of the patient as well. Before the beginning of a measurement, conventionally the operator of the magnetic resonance apparatus must confirm that any local antenna which is present is connected to the magnetic resonance apparatus and that no local antenna in an unconnected condition is located in the magnetic resonance apparatus.